1. Field of the Invention
The present invention concerns a computer-assisted detection method for diaphragm edges caused in an image by a diaphragm, of the type wherein the image is provided to a computer, the computer detects image edges present in the image, and the diaphragm edges are determined from the detected image edges.
2. Description of the Prior Art
In particular in the acquisition of x-ray images, the radiation beam path is normally blocked at the outer edge with shielding elements—for example, lead plates or the like. The radiation exposure for the examined subject (usually a person) is thereby minimized, and irradiation and/or scattered radiation effects are at least partially reduced. Furthermore, the data quantity is reduced. This concerns both the storage of the image data and its reproduction and processing.
The shielding elements form a diaphragm. The inner edges of the shielding elements are the diaphragm edges. They normally form a polygon, usually a rectangle, but the rectangle does not always exhibit the same orientation.
Relevant image information is located only within the inner (unblanked) region defined by the diaphragm edges.
Naturally, it is possible to acquire and display the entire image and then have a user enter an input representing the position of the diaphragm edges. For example, the user can plot these edge positions interactively in the displayed image, but this procedure is slow, laborious and error-prone.
Therefore, methods have been developed for computer-assisted detection of diaphragm edges by means of which the detection mentioned above is automatically possible. German OS 197 42 152, German OS 197 34 725 and U.S. Pat. No. 5,351,306 disclose examples of such methods.
The known methods operate quite satisfactorily in the majority of application cases. Given disadvantageous conditions, in particular when large implants are located in the patient to be examined or when the diaphragm contrast differs too little from the patient contrast, the diaphragm edges, however, are not recognized as such. It is thus possible that no diaphragm edge recognition ensues at all or that edges in the image of the examination subject are incorrectly determined as the diaphragm edges.